The present invention relates generally to the removal of particulate contaminants from commercial and industrial exhaust gases and pertains, more specifically, to an improvement in the construction and operation of electrostatic precipitators for attaining greater efficiency and effectiveness in removing such contaminants from a gas stream passed through an electrostatic precipitator, and especially from gas streams comprising high density mists and fumes containing submicron sized particles and droplets.
Electrostatic precipitators have been in use for a very long time in accomplishing the removal of particulates from gas streams. The principles which form the basis for the operation of electrostatic precipitators are well-known: Particulates entrained within a gas stream are subjected to an ionizing, or discharge voltage upon passing through an electrostatic field and are thereby charged so that the charged particulates will migrate, under the influence of the electrostatic field, in a direction generally perpendicular to the direction of flow of the gas stream, to be separated from the gas stream for collection and disposal. Among electrostatic precipitators in common use are single-stage devices in which an operating voltage is applied between a charging electrode and a collector electrode. The charging electrode charges the particulates in the gas stream and the operating voltage between the charging electrode and the collector electrode imparts a migration velocity to the charged particulates, causing the particulates to migrate toward the collector electrode for separation from the gas stream. Since separation efficiency is directly related to the magnitude of the migration velocity of the particulates, and the magnitude of the migration velocity is directly proportional to operating voltage, it becomes important to maintain the operating voltage as high as possible.
Typically, the particulates are charged by ionization induced between the charging electrode and the collector electrode, the ionization being facilitated by utilizing sharp points, provided by thin wires or pointed needle-like projections along the charging electrode. High operating current becomes essential in order to supply sufficient charge to the particulates and effect removal with efficiency. However, operating voltage is limited by the voltage at which a discharge occurs between the charging electrode and the collector electrode, commonly referred to as xe2x80x9csparkoverxe2x80x9d voltage, thereby limiting not only the operating voltage, but the operating current as well. This is true especially where the gas stream comprises a high density mist or fumes of submicron sized particles or droplets, all of which can reduce the voltage at which sparkover occurs.
Operating voltage can be increased considerably through the elimination of sharp-pointed projections so that the charging electrode is provided with a relatively smooth external surface; however, such a smooth surface reduces current flow and, consequently, reduces the charge supplied to the particulates, with the result that particulates no longer can be removed efficiently.
Past proposals for dealing with these conflicting requirements for high operating current, on the one hand, to achieve effective charging of particulates to be removed from a gas stream, and high operating voltage, on the other hand, to attain effective migration velocities for efficient removal of the charged particulates, have resulted in multiple-pass systems requiring relatively large and expensive installations.
The present invention provides an improvement which accomplishes the desired high operating current, for charging particulates, and high operating voltage, for separating and removing the charged particulates, in a simplified integrated single-pass electrostatic precipitator. As such, the present invention attains several objects and advantages, some of which are summarized as follows: Provides an integrated, relatively compact electrostatic precipitator and method for accomplishing increased effectiveness and efficiency in separating particulate contaminants from commercial and industrial exhaust gas streams; attains effective and efficient separation of particulates from gas streams such as high density mists and fumes containing submicron sized particles or droplets in a single electrostatic precipitator unit; provides a desired high operating current in a first electrostatic field for charging particulates, and a desired high operating voltage in a second electrostatic field for imparting migration velocity to the charged particulates to effect efficient separation of the particulates from a stream of gas passed through a single electrostatic precipitator; enables increased effectiveness and efficiency in the operation of an electrostatic precipitator, especially in dealing with particulates entrained in high density mists or fumes containing submicron sized particles or droplets; allows the construction of an electrostatic precipitator, and especially a condensing wet electrostatic precipitator, with increased economy and with more compact dimensions; enables the use of a single source of high voltage power in providing high operating current to a charging section of an integrated electrostatic precipitator, and high operating voltage to a collecting section of the integrated electrostatic precipitator for economy and efficiency in separating particulates from a gas stream passed through the integrated electrostatic precipitator; provides a wet electrostatic precipitator and, in particular, a condensing wet electrostatic precipitator, with a construction which utilizes relatively inexpensive corrosion-resistant materials, such as synthetic polymeric materials, for effective operation in connection with exhaust gases containing corrosive constituents; facilitates the attainment of condensation in a condensing wet electrostatic precipitator without the requirement for relatively heavy cooling structures ordinarily associated with condensing wet electrostatic precipitators; provides long-term, reliable operation in electrostatic precipitators effective in separating particulate contaminants from commercial and industrial exhaust streams and, in particular, exhaust streams which include high density mists or fumes of submicron sized particles or droplets.
The above objects and advantages, as well as further objects and advantages, are attained by the present invention which may be described briefly as an improvement in an electrostatic precipitator for removing particulate contaminants entrained in a stream of gas by passing the stream of gas in a downstream direction through an electrode arrangement in which the particulate contaminants are charged and subjected to an electrostatic field to be removed from the stream of gas and collected for further disposition, the improvement comprising: a charging section in the electrode arrangement for charging the particulate contaminants as the stream of gas passes through the electrode arrangement; a collecting section in the electrode arrangement located downstream from the charging section for collecting particulate contaminants charged in the charging section; the charging section including at least one charging electrode and a corresponding field electrode for charging the particulate contaminants; the collecting section including at least one collecting electrode for collecting charged particulate contaminants and a corresponding repelling electrode for driving the charged particulate contaminants toward the collecting electrode, the repelling electrode and the charging electrode being electrically separated from one another, and the collecting electrode being integral with the field electrode and located downstream of the field electrode such that the charging section and the collecting section comprise an integrated compact structure; a charging power source for providing a charging voltage and a charging current to the charging electrode; and a collecting power source for providing a collecting voltage to the repelling electrode at a voltage higher than the charging voltage and a current lower than the charging current, such that the charging section and the collecting section each are provided with a corresponding electrostatic field operating at an optimum voltage and current for respectively charging and collecting particulate contaminants entrained in the stream of gas.
In addition, the present invention provides an improvement in a method for removing particulate contaminants entrained in a stream of gas by passing the stream of gas in a downstream direction through an electrostatic precipitator having an electrode arrangement in which the particulate contaminants are charged and subjected to an electrostatic field to be removed from the stream of gas and collected for further disposition, the improvement comprising: charging the particulate contaminants in a charging section having at least one charging electrode and a corresponding field electrode as the stream of gas passes through the electrode arrangement; collecting, in a collecting section having at least one collecting electrode, charged particulate contaminants charged in the charging section and driven toward the collecting electrode by a repelling electrode; integrating the collecting electrode with the field electrode such that the charging section and the collecting section comprise an integrated compact structure; and electrically separating the repelling electrode from the charging electrode so as to enable: providing a charging voltage and a charging current to the charging electrode; and providing a collecting voltage to the repelling electrode at a voltage higher than the charging voltage and a current lower than the charging current, such that the charging section and the collecting section each are provided with a corresponding electrostatic field operating at an optimum voltage and current for respectively charging and collecting particulate contaminants entrained in the stream of gas.